The present invention is directed toward a method and apparatus for controlling the output of a transformer isolated, constant current, series resonant converter.
In general, in conventional resonant converter designs, the switching of a combination of transistors (or equivalents) transfers energy from the input bus through a tank capacitor. During the time the switches are ON energy is transferred to the load and the capacitor, and the capacitor voltage builds up. During the time the switches are OFF the energy is transferred from the capacitor and the secondary to the output load, and the capacitor voltage bleeds down. An earlier firing angle (switching time relative to current zero crossing) increases the amount of energy put into the tank capacitor for subsequent transfer to the load, and decreases the time for voltage to be bled off the capacitor. Conversely, a later firing angle decreases the energy put into the tank for eventual transfer to the load, but allows greater time for the capacitor voltage to bleed down.
Prior art control schemes thus control the output by modulating the times the switches are turned on and off. Modulation is relatively inexpensive and easy to implement, but has a significant draw back: the tank is not protected during operation, and increasing tank voltages and currents may result in component failure.
More specifically, if the load does not remove all the stored energy, the energy in the tank capacitor will grow each time the switches turn ON, resulting in a continuous increase of tank voltages and currents that will eventually exceed the safe operating ranges for the components in the resonant converter. If the unsafe operating condition persists, the components will fail.
One prior art controller may be found in The Miller Electric Co. XMT.RTM. power supply and controls the output of a resonant converter in response to information derived from the output load current and the current in the switches of the resonant converter. Generally, the controller causes the firing angle to initially be at a safe (i.e. later) angle, and then causes the firing angle to "creep" earlier. As the firing angle becomes earlier, the voltage and current are monitored. If they become dangerous the angle is immediately increased to a much safer (later) time.
This tank control scheme, while better than other control schemes, has several shortcomings. First, the response time to increase the output is slow because the firing angle creeps forward. In other words, the XMT.RTM. controller commands the converter switches to turn OFF before it is actually required in an attempt to remain in the safe operating range of the components. Thus, the energy stored in the tank capacitor and available for transfer to the load is not necessarily the maximum safe amount of energy. As a result, the response to transients is slow, and maximum output cannot be maintained.
Second, the components might not be optimally used nor adequately protected because the amount of energy stored in, and the voltage developed across, the tank capacitor is not relied on to turn the switches ON and OFF.
Third, the XMT.RTM. controller further avoids unsafe operating condition by using components that are overrated for normal operating conditions. The use of overrated components increases both the cost and physical size of the power converter. Despite this safeguard, occasional transients, which exceed the average anticipated transient, could possibly create voltages in excess of the safe operating range of the components. Thus, the reliability of the existing method is compromised because the switches in the resonant converter are often damaged or destroyed.
A very complex method of control used in other technical fields is optimal trajectory control. Optimal trajectory control is a control scheme that calculates the firing angle necessary to obtain a specific desired tank current and voltage. This type of control is difficult and expensive.
Specifically, optimal trajectory control selects the optimal trajectory from a range of trajectories, and thus requires the solving of complex, multi-variable, four dimensional equations. Moreover, the equations typically include derivatives and integrals and are highly nonlinear. The electronics necessary to solve such complex equations are expensive and difficult to use.
Accordingly, a tank controller for a series resonant converter that transfers the maximum safe amount of energy to the load is desirable. Additionally, such a controller should be inexpensive and not require the solving of complex, multi-variable, high order equations. Specifically, it is desirable to use the low cost, low complexity aspect of modulation control schemes, but avoid the unsafe operation that is inherently allowed by modulation. Conversely, the safe, protective aspect of trajectory control is desirable, while the cost and complexity should be avoided. Also, the controller will preferably be capable of preventing transients or other high voltages from damaging components.
In addition to protecting the tank, it is desirable to control the power supply output and provide a desired V-A curve. Typically, prior art inverter power supply output V-A curves include a constant voltage portion at currents much less than the setpoint and a sloped portion having increasing current as voltage decreases near the operating range. Also, some machines provide a "dig" where the slope increases (greater increase of current for a given decrease in voltage) for lower than normal voltage operation. However, it is desirable in some welding processes (such as stick welding) to provide a constant current output.
Thus, it is desirable to provide a power supply that has a constant current portion of the output V-A curves, particularly at typical welding voltages. Additionally, it would be desirable to provide an adjustable slope dig.
Some prior art machines provide a boost of energy when the welding process is started, called a hot start. The hot start allows arc ignition without sticking of the arc. Typically, the excess energy provided by a power supply for a hot start was of fixed amplitude and duration. However, skilled welders typically require less energy for starting than beginners do. Thus, prior art hot starts were too hot for some users, and not hot enough for others. Accordingly, a tunable or adaptive hot start is desired.